1. Field of the Invention
The present invention generally relates to an optical recording method for recording information on an optical recording medium such as a rewritable compact disc (CD-RW) or a rewritable digital video disc (DVD-RW) and, more particularly, to an optical recording method for recording information on a rewritable phase-change optical recording medium by projecting an optical beam onto the rewritable phase-change optical recording medium so as to cause a change of phase in a recording layer of the optical recording medium.
2. Description of the Related Art
A phase-change optical recording medium is known as an optical memory medium onto which information is recorded or information recorded thereon is reproduced or erased by projecting a laser beam onto the optical memory medium. The phase-change optical recording medium utilizes a transformation between a crystal phase and a noncrystal phase or between a crystal phase and a different crystal phase. Development of the phase-change optical recording medium has become active since information recorded on the phase-change optical recording medium can be overwritten by using a single laser beam and an optical system of a drive unit can be a relatively simple structure.
A typical phase-change optical recording medium is disclosed in U.S. Pat. No. 3,530,441. The phase-change optical recording medium disclosed in this patent document is made of a so-called chalcogenic alloy such as Gexe2x80x94Te, Gexe2x80x94Texe2x80x94Sn, Gexe2x80x94Texe2x80x94S, Gexe2x80x94Sexe2x80x94Sb, Ge-Asxe2x80x94Se, Inxe2x80x94Te, Sexe2x80x94Te or Sexe2x80x94As. Each of Japanese Laid-Open Patent Applications No. 61-219692, No. 61-270190 and No. 62-19490 suggests a material in which Au and Sn or Au and Pd are added to a Gexe2x80x94Te alloy for improving a stability and a high-speed crystalization of the material. Additionally, each of Japanese Laid-Open Patent Applications No. 62-73438 and No. 63-228433 suggests a Gexe2x80x94Texe2x80x94Sexe2x80x94Sb material or a Gexe2x80x94Texe2x80x94Sb material having a composition ratio which is specifically defined for improving a repetition characteristic of recordings and erasures. However, none of the materials suggested in the above-mentioned patent documents can satisfy all of the characteristics required for a rewritable phase-change optical recording medium. An important issues to be solved are 1) improvement in recording sensitivity and erasing sensitivity, 2) prevention of decrease in an erasure ratio due to incomplete erasure during an overwriting operation and 3) extension of a service life of a recordable part and a nonrecordable part.
Additionally, Japanese Laid-Open Patent Application No. 63-251290 suggests a recording medium provided with a recording layer comprising a single, multi-state composite layer of which crystal state can be substantially more than three different states. Such a multi-state composite layer which can be more than three different states is defined as a composite layer which includes more than 90 atomic percentage of a composite material (for example, In3SbTe2) having a chemical composite of more than three states. By using such a recording layer, a recording and erasing characteristic may be improved. However, there still exist problems in that an erasure ratio is small and a laser power used for recording and erasure is not sufficiently reduced.
Further, Japanese Laid-Open Patent Application No. 1-277338 suggests an optical recording medium having a recording layer made of an alloy having a composite represented by (SbaTe1-a)1-yMy, where 0.4:xe2x89xa6axe2x89xa60.7, yxe2x89xa6=0.2 and M is at least one element selected from a group consisting of Ag, Al, As, Au, Bi, Cu, Ga, Ge, In, Pb, Pt, Se, Si, Sn and Zn. The basic material of this system is Sb2Te3. A high-speed erasure characteristic and a repetition characteristic are improved by providing an excessive amount of Sb, and the high-speed erasure characteristic is promoted by adding the element M. Additionally, it is explained that a large erasure ratio by a DC light can be achieved. However, this patent document does not teach an erasure ratio during an overwriting operation, and a recording sensitivity is insufficient.
Additionally, Japanese Laid-Open Patent Application No. 60-177446 suggests an optical recording medium including a recording layer made of an alloy having a composite represented by (In1-xSbx)1-yMy, where 0.4xe2x89xa6axe2x89xa60.7, yxe2x89xa60.2 and M is at least one element selected from a group consisting of Au, Ag, Cu, Pd, Pt, Al, Si, Ge, Ga, Sn, Te, Se and Pi. Japanese Laid-Open Patent Application No. 63-228433 teaches a Gexe2x80x94Texe2x80x94Sb2Te3xe2x80x94Sb(excessive) alloy used for a recording layer. However, the recording layer disclosed in each of the above-mentioned two patent documents does not satisfy a sensitivity characteristic and an erasure ratio.
Further, Japanese Laid-Open Patent Application No. 4-16383 discloses an optical recording medium provided with a recording thin film formed of a Texe2x80x94Gexe2x80x94Sb alloy containing N. Japanese Laid-Open Patent Application No. 4-52188 discloses an optical recording medium provided with a recording thin film formed of a Texe2x80x94Gexe2x80x94Se alloy containing a nitride of at least one of the component elements of the alloy. Japanese Laid-Open Patent Application No. 4-52189 discloses an optical recording medium provided with a recording thin film formed of a Texe2x80x94Gexe2x80x94Se alloy which is caused to adsorb N. However, none of the above-mentioned optical recording media can provide sufficient characteristics.
As discussed above, in a conventional optical recording medium, the most important issues to be solved are 1) improvement in recording sensitivity and erasing sensitivity, 2) prevention of decrease in an erasing ratio due to incomplete erasure during an overwriting operation and 3) extension of a service life of a recordable part and a nonrecordable part.
On the other hand, in association with a rapid spread of a compact disc (CD), a recordable compact disc (CD-R), which is recordable only once, was developed and has become popular. However, the CD-R cannot correct recorded information when a recording operation has failed. Accordingly, the failed disc is not reusable and must be discarded. Thus, it is desirous to develop a rewritable compact disc so as to eliminate such a drawback.
A rewritable compact disc which utilizes a magneto-optical disc has been developed. However such a rewritable compact disc has drawbacks in that an overwriting operation is difficult to perform and there is no compatibility with a CD-ROM and a CD-R. Accordingly, instead of the rewritable compact disc utilizing a magneto-optical disc, development of a phase-change optical disc has become active.
The following technical documents are related to a rewritable compact disc utilizing a phase-change optical disc:
1) Fourth phase change recording study symposium preceding, 70 (1992), K. Furuya et al., xe2x80x9cDisk structure and composition of Gexe2x80x94Texe2x80x94Sb recording film suitable for low linear velocityxe2x80x9d
2) Fourth phase change recording study symposium preceding., 76 (1992), S. Jinno et al., xe2x80x9cErasable Phase Change Optical Disks for Recording at Low linear Velocityxe2x80x9d
3) Fourth phase change recording study symposium preceding, 82 (1992), Y. Kawanishi et al., xe2x80x9cWrite, Read and Erase Characteristic of InSe Phase Change CDxe2x80x9d
4) Jpn. J. Appl. Phys., 32 (1993), T. Handa et al.
5) Fifth phase change recording study symposium preceding, 9 (1993), S. Maita et al., xe2x80x9cErasable Phase Change Optical Disksxe2x80x9d
6) Fifth phase change recording study symposium preceding, 5 (1993), J. Tominaga et al., xe2x80x9cOverwrite Properties of V doped Inxe2x80x94Agxe2x80x94Texe2x80x94Sb Optical Phase Change Media at CD Velocityxe2x80x9d
However, none of the phase-change optical discs satisfies the desirous characteristics such as compatibility with a CD-ROM and a CD-R, a recording and erasing performance, a recording sensitivity, a number of times of repetition of overwriting, a number of times of reproduction, a stability for storage or a total performance. The drawbacks are mainly caused by a small erasure ratio due to a composition and structure of a material of the optical disc.
Accordingly, it is desirous to develop a phase-change recording material which is suitable for a high-speed recording and erasure. Additionally, it is desirous to develop a rewritable phase-change compact disc having a high performance.
The inventors have suggested an AgInSbTe recording material to be used as a new material that eliminates the above-mentioned drawbacks. A typical material suggested by the inventors is disclosed in the following technical documents:
1) Japanese Laid-Open Patent Application No. 4-78031
2) Japanese Laid-Open Patent Application No. 4-123551
3) Jpn. J. Appl. Physics., 31 (1992), H. Iwasaki et al.
4) Third phase change recording study symposium preceding, 102 (1991), Y. Ide et al., xe2x80x9cCompletely Erasable Phase Change Optical Diskxe2x80x9d
5) Jpn. J. Appl. Physics., 32 (1993), 5242
Additionally, a standard for specifying a rewritable compact disc, which standard may be widely used in this field, was issued in October, 1996. Hereinafter, this standard may be referred to as a conventional standard.
The above-mentioned conventional standard is related to a CD-RW for a linear velocity recording of a double speed (2xc3x97) (2.4 to 2.8 m/s). However, in such a low-speed recording, a recording time is long. Thus, it is desirous to develop a rewritable compact disc suitable for a high-speed recording. On the other hand, the CD-RW is expected to be used as an audio disc that is driven at a normal speed (1xc3x97). Accordingly, it is desirous that a single CD-RW be used for a multi-speed recording at a double speed (2xc3x97) and a four-times (4xc3x97) speed, and for a recording at the normal speed (1xc3x97).
In a phase-change optical recording medium to which a constant angular velocity (CAV) recording and reproducing method is, applied, such as a DVD-RAM or a DVD-RW, a recording Linear velocity at an outer portion of the disc may be more than two times as fast as a recording linear velocity at an inner portion of the disc. The optical recording medium is preferably formed with a uniform layered-structure and a uniform substrate structure from an innermost portion to an outermost portion. That is, it is desirous that a single optical recording medium be used for a multi-speed recording including a normal speed (1xc3x97), a double speed (2xc3x97) and a four-times speed (4xc3x97).
According to the above-mentioned technologies, it is considered that a phase-change optical recording medium having an extremely superior performance can be achieved by using an AgInSbTe recording material for a recording layer. However, in order to increase a range of an effective linear velocity for recording and satisfy a desired total performance so as to create a new rewritable optical disc market, an improvement in the recording technique (optical recording method) is required as well as development of a material used for a phase-change optical recording medium.
It is a general object of the present invention to provide an improved and useful optical recording method in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an optical recording method suitable for a phase-change optical recording medium which can have information recorded thereon by a multi-speed recording, in which method both a low-linear-velocity recording and a high-linear-velocity recording can be effectively performed.
In order to achieve the above-mentioned objects, there is provided according to the present invention an optical recording method for recording information on a phase-change optical recording medium which is recordable at a low linear velocity and a high linear velocity greater than the low linear velocity, the optical recording method comprising the steps of:
determining a ratio (Pel/Pwl) of an erasing power Pel to a recording power Pwl in the low-linear-velocity recording and a ratio (Peh/Pwh) of an erasing power Peh to a recording power Pwh in the high-linear-velocity recording so that the ratio (Pel/Pwl) and the ratio (Peh/Pwh) satisfy the relationship {(Pel/Pwl) less than (Peh/Pwh)}; and
recording information on the phase-change optical recording medium and/or erasing information recorded on the phase-change optical recording medium by causing a change of phase in a recording layer of the phase-change optical recording medium by projecting a light beam onto the phase-change optical recording medium according to the recording power and the erasing power determined in the determining step.
According to the present invention, the ratio of the erasing power Pel to the recording power Pwl in the low-linear-velocity recording is rendered to be smaller than the ratio of the erasing power Peh to the recording power Pwh in the high-linear-velocity recording. Accordingly, a residual heat generated by application of the erasing power in the low-linear-velocity recording is reduced, which results in recording of an appropriate recording mark. As a result, a signal characteristic of a reproduction signal in the low-linear-velocity recording is improved. Additionally, since an excessive laser power is not applied to the recording layer, the overwrite characteristic of the optical recording medium can be well-maintained or even improved.
Preferably, the ratio (Pel/Pwl) and the ratio (Peh/Pwh) satisfy the relationship {(Pel/Pwl)=Axc3x97Peh/Pwh)}, where Axe2x89xa60.9.
Additionally, the ratio (Pel/Pwl) and the ratio (Peh/Pwh) may satisfy the relationship {(Pel/Pwl)=Bxc3x97(Peh/Pwh)}, where Bxe2x89xa70.45.
Further, the optical recording method according to the present invention may further include the step of setting an Asymmetry (Asym) in the low-linear-velocity recording and in the high-linear-velocity recording so that the Asymmetry (Asyml) in the low-linear-velocity recording and the Asymmetry (Asymh) in the high-linear-velocity recording satisfy the relationship (Asyml less than Asymh), where the Asymmetry (Asym) is a characteristic value defined by the following equation;
Asym=[(3TH+3TL)/2xe2x88x92(11TH+11TL)/2]+(11THxe2x88x9211TL)
where 3TH is a maximum value of an amplitude of a reproduction signal obtained from a shortest recording mark, 3TL is a minimum value of an amplitude of the reproduction signal obtained from the shortest recording mark, 11TH is a maximum value of an amplitude of a reproduction signal obtained from a longest recording mark, 11TL is a minimum value of an amplitude of the reproduction signal obtained from the longest recording mark.
According to this invention, a recording mark having an appropriate shape can be recorded in the low-linear-velocity recording. As a result, a signal characteristic of a reproduction signal in the low-linear-velocity recording is improved.
In the above-mentioned invention, the symmetry (Asyml) in the low-linear-velocity recording preferably satisfies the relationship (Asymlxe2x89xa6xe2x88x923.0).
Additionally, the optical recording method according to the present invention may further include the step of further determining the erasing power Pel and the recording power Pwl in the low-linear-velocity recording and the erasing power Peh and the recording power Pwh in the high-linear-velocity recording so that a difference (Pwlxe2x88x92Pel) and a difference (Pwhxe2x88x92Peh) satisfy the relationship {(Pwlxe2x88x92Pel) greater than (Pwhxe2x88x92Peh)}.
According to this invention, a cooling effect for a recording mark is increased, and, thereby, an appropriate recording mark can be recorded on the optical recording medium.
In this invention, preferably, the difference (Pwlxe2x88x92Pel) and the difference (Pwhxe2x88x92Peh) satisfy the relationship {(Pwlxe2x88x92Pel)=Cxc3x97(Pwhxe2x80x94Peh)}, where Cxe2x89xa71.1.
Additionally, the difference (Pwlxe2x88x92Pel) may be set to be equal to or greater than 7.2 mW.
In the optical recording method according to the present invention, a residual heat adjusting pulse may be provided for applying a residual heat adjusting power smaller than the erasing power Pel in the low-linear-velocity recording, the residual heat adjusting pulse preceding a front pulse according to a recording strategy for applying the recording power Pwl so as to record a recording mark.
Accordingly, in the low-linear-velocity recording, a residual heat in the vicinity of the start point can be removed by providing the residual heat adjusting pulse. Thus, an appropriate recording mark can be recorded on the optical recording medium in the low-linear-velocity recording.
Additionally, in the optical recording method according to the present invention, a recording layer of the phase-change optical recording medium may contain elements Ag, In, St, Te and N or 0, and respective composition ratios xcex1, xcex2, xcex3, xcex4 and xcex5 of the elements may satisfy the following relationships:
0 less than xcex1xe2x89xa66, 3xe2x89xa6xcex2xe2x89xa615, 50xe2x89xa6xcex3xe2x89xa665,
20xe2x89xa6xcex4xe2x89xa635, 0xe2x89xa6xcex510
xcex1+xcex2+xcex3+xcex4+xcex5xe2x89xa6100
Further, in the optical recording method according to the present invention, the low linear velocity may be set to 1.2 m/s to 1.4 m/s, and the high linear velocity may be set equal to or more than 2.4 m/s.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.